This is what you want but please call and speek with a sale's rep to order. As the guide I have linked you to is for the 378 and not a 30-378. They do have a 30 cal bore guide as well but I do not know enough about the MK V action to say it will work.

Try editing you post title to read I need Weatherby MK V help or something like that. There are 2 men here I know of using this action Tyler kemp is one he may have some advice for you.